How do I rotate the viewport, without leaving the x/y-plane?
Updated based on the comment:

The option you look for is the "Orbit left" using the Numpad 4 and "Orbit Right" using Numpad 6 respectively. At least in my logic that is the only way to rotate the viewport(as in the title of the question) and is not leaving the xy-plane. Not sure how "Roll and Orbit is different here as the result seems to be the same.


The increase or decrease in angle is set default to 15 degree in the preferences here:


Setting the Rotation Angle to 1.0, would result in the "Orbiting" to be a lot smoother.

Here a little visual demonstration:


Happy Blending